Presently, technologies for connecting semiconductor chips to underlying conductor arrays include thermo-compression and solder-based bonding. Thermo-compression bonding is a low cost process and is used for many low-to-mid performance chip applications. It requires the application of significant pressure to form the chip/conductor interconnection and may, at times, damage underlying active chip structures. Generally, this bonding method is not usable with chips which have active circuitry beneath the bonding pads. Its use with chips having perimeter I/O and area I/O interconnections significantly reduces the area of silicon available to accommodate circuitry and bonding pads and increases the chip's cost. Solder-based technologies have been used to interconnect to such chip structures, as they provide low stress bonding methods and enable circuitry to be located under the bonding pads. However, solder-based technologies are relatively expensive and typically have been limited to high-end packaging applications.
Efforts have been expended to establish chip connection by writing connector lines using thick film pastes similar to those used in hybrid-circuit technology. These pastes are typically suspensions of 40%-80% metal flakes in a polymer, which polymer is cured (i.e., not volatilized) to form metallic lines. While it is preferred to produce metallic lines whose resistivity approaches that of the pure metal, because some of the paste remains in the metallic lines, substantially higher resistivities are found.
Another prior art technique for providing conductive circuit lines employs chemical vapor deposition of a volatile organometallic complex on a substrate through the use of a laser to "write" lines on the substrate. Since this procedure is a direct "write" technique, it is throughput limited and does not lend itself to high volume production. Circuit lines had also been written with lasers by reducing a complex in a thermoplastic polymer. In none of these cases is a direct bond formed. Each is employed to create circuit lines either by laser deposition or by fine line printing of thick film pastes. These efforts are covered in more detail in the following references:
A. Auerbach, "Method for Reducing Metal Salts Complexed in a Polymer Host with a Laser, " J. Electro Chem. Soc., 132, 6, p. 1437, 1985.
M. Ohuchi et al., "Planar LSI Interconnection Method Utilizing Polymeric Conductor:, IMC 1986 Proceedings, Kobe, May 28-36, 1986.
Auerbach, "A New Scheme for Device Packaging", IEEE Transactions on Components, etc, "Vol CHMT-8, No. 3, Sep. 1985, pp. 309-312.
Hsu et al., "The Wire Bondability of Thick Film Gold, Sputtered Thin Film Gold and Metallo-Organic Thin Film Gold" Proceedings 1985 International Symposium on Microelectronics, Int. Soc. Hybrid Microelectronics, Anaheim, Cal., 11-15 Nov. 1985, pp. 428-434.
C. Needes et al., "Thick Film Materials for Copper Hybrid Circuits", IMC 1986 Proceedings, Kobe, May 28-30, 1986.
F. Houle, C. Jones, T. Baum, C. Pico, C. A. Kovac, "Laser Chemical Vapor Deposition of Copper", Appl. Phys. Lett., 46, p. 204, 1985.
T Baum and C Jones, "Laser Chemical Vapor Deposition of Gold", Appl. Phys. Lett. 47, p. 538, 1985.
Certain aspects of the above-noted deposition techniques present difficulties when applied to semiconductor applications. For instance, in order to drive off the pastes used to carry the metal flakes, high temperatures must be employed. These temperatures tend to damage and otherwise unfavorably affect circuit components, while still leaving the organic material incorporated in the final conductive lines.
It is known that certain organometallics reduce to a metal state at much lower temperatures than paste/polymer combinations (e.g. on the order of 100.degree.-300.degree. C. as contrasted to 800.degree. C.-1500.degree. C.). Organometallics have been used in the prior art to create metal seeding layers for subsequently applied conductor patterns. For instance, in U.S. Pat. No. 4,574,095 to Baum et al., a palladium organometallic compound is irradiated selectively by light, thereby depositing palladium seeds in the irradiated areas. Following the deposition of the palladium seeds, copper is deposited thereon.
In U.S. Pat. No. 4,701,351 to Jackson, a thin organometallic layer is applied as a coating and then may have an additional coating of a photoresist placed thereover. During subsequent processing, portions of the organometallic are reduced to the metal state, to provide a substrate upon which subsequent conductor deposition can occur.
In U.S. Pat. No. 4,734,481 to Steinmann, a class of organometallic polymers is described, particularly useful for photoactive coating agents for various types of substrates. The preferred organometallic polymer is an end-capped polyphthalaldehyde. These materials are employed similarly to other photosensitive photomicrography materials. In other words, they are applied as a layer, imaged in a conventional manner and the irradiated areas decomposed, with resultant monomers evaporating and exposing the underneath substrate. A benefit deriving from the use of these organometallics is that they convert to their volatile monomeric state at relatively low processing temperatures and leave little or no residue.
It is therefore an object of this invention to provide an interconnection technique which employs low temperatures and provides good metallurgical results.
It is another object of this invention to provide a low temperature interconnection technique which enables the generation of a homogenous metallic bonds.
It is a further object of this invention to provide a method of forming metal-metal bonds which are characterized by low stress, low temperature conditions.